A communication system, which performs communication between mobile communication devices without passing through a base station, such as communication between the cars, is known. In this communication system, when the communication device moves, a communication partner may leave from a communication area or a number of communication partners may decrease. Also, in an area where a plurality of communication devices concentrate, the number of partners of communication within the communication area may become very large. Moreover, these situations temporally fluctuate. In particular, in the area where the communication devices are concentrated, wireless communication is crowded and congestion of communication occurs, and a throughput declines.
Patent document 1 discloses a wireless communication device and a transmission power control method for selecting one among respective pieces of ID information of a plurality of in-vehicle communication devices received from a plurality of surrounding in-vehicle communication devices, acquiring a transmission power setting value and a receiving power measured value corresponding to the ID information, and determining a transmission power setting value for the next time with reference to a predetermined table. Data on the table may be deleted if there is no update for a scheduled time. In this related art, a communication possible area is adjusted by control of the transmission power. That is, the number of communication devices within the communication area is limited. As a result, a radio link with a surrounding communication device is secured. Also, the occurrence of congestion of communication is suppressed, and the decline in the throughput is reduced.
Patent document 2 discloses a communication terminal and a data communication method for determining a transmission condition of the transmission radio wave transmitted to other communication terminals based on running environment information at a current spot of a vehicle and on running speed information representing running speed of the vehicle. Further, the communication terminal disclosed in the patent document 2 transmits the running environment information and the running speed information to a fixedly installed information processing apparatus, and makes the information processing apparatus determine the transmission condition of the transmission radio wave transmitted to other communication terminals.